Subsea mudlift pump drilling is used in sub-bottom wellbore drilling in selected water depths to enable maintaining a fluid pressure and pressure gradient in the wellbore that is different than would be the case with conventional drilling, wherein drilling fluid pumps located on a drilling unit above the water surface pump drilling fluid into the well at such rates and pressures as to enable lifting the drilling fluid all the way from the bottom of the wellbore and back to the drilling unit above the water surface. As is known in the art, in conventional drilling the fluid pressure in the wellbore and pressure gradient are related to the pressure of the drilling fluid being pumped at the surface, the depth of the wellbore and the specific gravity (“mud weight”) of the drilling fluid.
It is known in the art to use a pump in the drilling fluid return line to the drilling unit above the water surface to reduce the fluid pressure and pressure gradient in the wellbore annulus (the space between the drill string and the wall of the wellbore) so that drilling may proceed to greater depths before the need to set a protective liner or casing in the wellbore. Such “subsea mudlift drilling” techniques may enable having a larger diameter wellbore at the planned total wellbore depth because fewer concentrically placed protective casings or liners may be needed than when using conventional drilling techniques. One example of such technique is described in U.S. Pat. No. 7,677,329 issued to Stave and incorporated herein by reference. One limitation to subsea mudlift pump systems such as those described in the Stave patent is that the upper portion of the drilling riser is open, and is frequently filled with air above the maintained level of drilling fluid in the riser. While essentially all drilling fluid pumped into the wellbore is returned to the drilling unit by the subsea mudlift pump, safety considerations suggest the need to ensure that an explosive atmosphere, caused by entrained wellbore gas in the returning drilling fluid entering the air-filled portion of the riser through the open wellbore connection to the riser, does not come into existence.
What is needed is a system for inhibiting accumulation and/or maintenance of explosive atmospheres in open top risers using subsea mudlift systems such discussed above.